1. Field of the Invention
The present invention relates to a switching electric source device including a transformer and a circuit for rectifying electric power output from a secondary winding of the transformer by use of a synchronous rectifier.
2. Description of the Related Art
A switching electric source device has been proposed which is provided with a detection circuit for indirectly detecting an output voltage, and controls a main switching element based on detection results by the detection circuit according to a PWM system (e.g., see Japanese Unexamined Patent Application Publication No. 2001-25245 (Patent Document 1)). FIG. 9 shows an example of a circuit configuration of major components of the switching electric source device having the detection circuit described above.
The switching electric source device 200 contains a transformer 1. A main switching element (MOSFET) Q1 is connected to the primary winding N1 of the transformer 1. The circuit containing the transformer 1 and the main switching element Q1 connected in series is connected to an external input electric source 5 via an input filter 6.
A secondary rectifying smoothing circuit 20 is connected to a secondary winding N2 of the transformer 1. The secondary rectifying smoothing circuit 20 includes a first synchronous rectifier Q2 (switching element (MOSFET)) connected in series with the secondary winding N2, a second synchronous rectifier Q3 (switching element (MOSFET)) connected in parallel to the secondary winding N2, and a circuit containing a choke coil 21 and a capacitor 22 connected in series, the circuit containing the choke coil 21 and the capacitor 22 being connected in parallel to the synchronous rectifier Q3. The secondary rectifying smoothing circuit 20 rectifies output power from the secondary winding N2, using the switching operation of the synchronous rectifiers Q2 and Q3, smoothes the output power via the choke coil 21 and the capacitor 22, and outputs DC voltage Vout to an external load.
A second synchronous rectifier drive circuit 25 is connected to the switching control terminal (gate terminal) of the synchronous rectifier Q3. The second synchronous rectifier drive circuit 25 controls the switching operation of the synchronous rectifier Q3 such that the synchronous rectifier Q3 carries out the switch on-off operation inverting that of the main switching element Q1. In particular, when the main switching element Q1 is off, the synchronous rectifier Q3 is on, and when the main switching element Q1 is on, the synchronous rectifier Q3 is off, caused by the second synchronous rectifier drive circuit 25.
Moreover, the synchronous rectifier Q2 carries out the same switch on-off operation as that of the main switching element Q1, using the induced voltage of the secondary winding N2.
A third winding N3 is provided for the transformer 1. A detection circuit 30 is connected to the third winding N3. The detection circuit 30 includes diodes 31 and 32 which are rectifying elements for rectifying voltage induced in the third winding N3, a choke coil 33 and a capacitor 34 for smoothing the voltage, and voltage-dividing resistors 35 and 36 for dividing the rectified, smoothed voltage. The voltage corresponding to voltage induced in the secondary winding N2 is induced in the third winding N3. Thus, the detection circuit 30 rectifies and smoothes the induced voltage in the third winding N3, and thereby, indirectly detects the output voltage Vout which is output from the secondary rectifying smoothing circuit 20 to a load, and outputs the detection voltage with respect to the output voltage Vout.
A control circuit 10 is connected to the gate terminal as a switching control terminal of the main switching element Q1. The control circuit 10 controls the switch on-off operation of the main switching element Q1 based on the detection voltage according to a PWM system. The control circuit 10 includes an error amplifier 11, a reference voltage source 12, a comparator 13, and a triangular-wave signal oscillator 14. In particular, in the control circuit 10, the error amplifier 11 amplifies an error voltage between the detection voltage with respect to the output voltage Vout obtained in the detection circuit 30 and the reference voltage output from the reference voltage source 12. The comparator 13 compares the amplified voltage to the magnitude of a triangular-wave signal output from the triangular-wave signal oscillator 14. Thus, a switching-control signal (a pulse signal) generated based on the comparison results is applied to the gate terminal of the main switching element Q1. When a switching control signal is on a high level, the main switching element Q1 turns on. When the switching control signal is on a low level, the main switching element Q1 turns off. As described above, the switching control for the main switching element Q1 is carried out by the control circuit 10 based on detection results obtained in the detection circuit 30 which indirectly detects the output voltage Vout output from the secondary rectifying smoothing circuit 20.
Hereinafter, an example of the operation of the main circuit components of the switching electric source device 200 will be described with reference to waveform examples shown in FIGS. 10A to 10F.
For example, when the main switching element Q1 is on, which is caused by the control-operation of the control circuit 10 (e.g., an ON time period shown in FIGS. 10A to 10F), an input voltage Vin, input from the input electric source 5, is smoothed by the input filter 6 and supplied to the primary winding N1. Thereby, voltage is induced in the secondary winding N2. The induced voltage in the secondary winding N2 causes the synchronous rectifier Q2 to turn on (see FIG. 10D), and moreover, the synchronous rectifier Q3 is caused to turn off by the second synchronous rectifier drive circuit 25 (see FIG. 10C). The switching operation of the synchronous rectifiers Q2 and Q3 causes current to flow on the secondary side along a current loop from the secondary winding N2 via the load, the choke coil 21, and the synchronous rectifier Q2 to the secondary winding N2, so that an output voltage Vout is output to the load. With this current-conduction, magnetizing energy is stored in the choke coil 21.
Moreover, while the main switching element Q1 is in the ON time period, the current based on the induced voltage of the third winding N3 flows through the detection circuit 30 along a current-loop from the third winding N3 via the voltage-dividing resistors 35 and 36, the control circuit 10, the choke coil 33 and the diode 32 to the third winding N3. Thus, the detection circuit with respect to the output voltage Vout is output from the detection circuit 30 to the control circuit 10. With this current-flow, the exciting energy corresponding to the output voltage Vout is stored in the choke coil 33.
When the main switching element Q1 is off (e.g., the OFF time period in FIGS. 10A to 10F), the synchronous rectifier Q2 is off, and the synchronous rectifier Q3 is on. Thereby, the exciting energy stored in the choke coil 21, as current, conducts along a current loop from the choke coil 21 via the synchronous rectifier Q3 and the load to the choke coil 21, so that the output voltage Vout is output to the load. Then, in the detection circuit 30, the detection current with respect to the output voltage Vout and based on the exciting energy of the choke coil 33 flows along a current loop passing through the choke coil 33, the diode 31, the voltage-dividing resistors 35 and 36, and the control circuit 10. Thus, the detection voltage with respect to the output voltage Vout is output from the detection circuit 30.
When the main switching element Q1 is switched off, resonance occurs due to the primary winding N1 and the parasitic capacitance of the main switching element Q1 based on the exciting energy of the primary winding N1, as shown by a time period B in FIGS. 10A to 10F. During the time period (a time period A) ranging from the completion of the resonance operation to the switching on of the main switching element Q1, the exciting current of the secondary winding N2 flows along a current loop of from the parasitic diode (body diode) 23 of the synchronous rectifier Q2 via the secondary winding N2 and the synchronous rectifier Q3 to the body diode 23, so that the excitation of the secondary winding N2 can be maintained due to the existence of the parasitic diode (body diode) 23 between the drain-source of the synchronous rectifier Q2. Thus, the exciting current of the secondary winding N2 has no relationship to the output voltage Vout, since the exciting current does not flow through the choke coil 21.
The current flows through the secondary winding N2 via the body diode 23 as described above. The voltage corresponding to the voltage drop in the body diode 23 is induced in the secondary winding N2. The secondary winding N2 and the third winding N3 are magnetically coupled to each other. Thus, voltage VN3A corresponding to the induced voltage in the secondary winding N2 is induced in the third winding N3 during the time period A (e.g., see FIG. 10E). Current based on the induced voltage VN3A of the third winding N3 flow along a current loop of from the choke coil 33 via the diode 32, the third winding N3 the voltage-dividing resistors 35 and 36, the control circuit 10 to the choke coil 33. Thus, the current is superposed on the detection current with respect to the output voltage Vout and based on the exciting energy for the choke coil 33.
The voltage VN3A induced in the third winding N3 during the time period A in the OFF time period of the main switching element Q1 (from the completion of the resonance operation on the primary side to the switching on of the main switching element Q1) can be expressed by the following numerical formula: VN3A=Vf×(N3/N2) in which Vf represents the voltage drop in the body diode 23 of the synchronous rectifier Q2, N2 represents the number of turns of the secondary winding N2, and N3 represents the number of turns of the third winding N3. During the time period A in which the induced voltage VN3A is generated, the current based on the induced voltage VN3A, and the detection current with respect to the output voltage Vout and based on the exciting energy of the choke coil 33 flows in the detection circuit 33. Thus, the voltage rectified in the detection circuit 30 is equal to the sum of the voltage between the both ends of the choke coil 33, the voltage drop in the diode 32, and the induced voltage VN3A in the third winding N3. However, ordinarily, the impedance of the control circuit 10 is considerably higher than that of the detection circuit 30. Therefore, the both-end voltage of the choke coil 33 during the time period A is lower than the both-end voltage of the choke coil 33 during the time period B in an amount equal to the induced voltage VN3A of the third winding N3 which is due to the voltage drop in the body diode 23 of the synchronous rectifier Q2 (e.g., see FIG. 10F).
The current flowing in the detection circuit 30 based on the induced voltage VN3A of the third winding N3 during the time period A in the OFF time period of the main switching element Q1 corresponds to the exciting energy of the secondary winding N2, and is independent of the output voltage Vout. Thus, a problem occurs in that the current is superposed on the detection current with respect to the output voltage Vout and based on the exciting energy of the choke coil 33, and thus, the correct detection voltage with respect to the output voltage Vout can not be obtained by the detection circuit 30. The voltage drop Vf in the body diode 23 during the OFF time period of the synchronous rectifier Q2 is significantly large compared to that during the ON time period. Thus, the current superposed on the detection current with respect to the output voltage Vout of the detection circuit 30 is not negligible. This deteriorates the detection accuracy of the output voltage Vout of the detection circuit 30.
The time period A in the OFF time period of the main switching element Q1 becomes longer as the input voltage Vin increases. The longer the time period A is, the lower the detection accuracy of the output voltage Vout obtained by the detection circuit 30 is. Therefore, the output voltage Vout to the load is reduced due to the switching control of the main switching element Q1 which is carried out by the control circuit 10 based on the detection voltage of the detection circuit 30. Moreover, the higher the ambient temperature is, the larger the voltage drop Vf in the body diode 23 of the synchronous rectifier Q2 is. Accordingly, the lower the ambient temperature is, the lower the detection accuracy of the detection circuit 30 with respect to the output voltage Vout is, and thus, the output voltage Vout to the load becomes lower. It is desirable to have an output voltage characteristic which is constant irrespective of the input voltage Vin and a change in the ambient temperature, as shown by solid line a in the graph of FIG. 11. According to the configuration of the switching electric source device 200 shown in FIG. 9, the following output voltage characteristic is obtained: the output voltage Vout becomes lower, as the input voltage Vin becomes higher, and moreover, the ratio of the variation of the output voltage Vout to that of the input voltage Vin increases as the ambient temperature becomes lower, which is shown by dotted lines b, c, and d in the graph of FIG. 11.
In some cases, an external diode is provided between the drain-source of the synchronous rectifier Q2. Also, the above-described problems occur.